Re: [PATCH] Re: boot time, process start time, and NOW time

[George Anzinger]

Tim Schmielau wrote:
> On Mon, 16 Aug 2004, Andrew Morton wrote:
>
>
> > OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx> wrote:
> >
> > > Albert Cahalan <albert@xxxxxxxxxxxx> writes:
> > >
> > >
> > > > Even with the 2.6.7 kernel, I'm still getting reports of process
> > > > start times wandering. Here is an example:
> > > >
> > > >   "About 12 hours since reboot to 2.6.7 there was already a
> > > >   difference of about 7 seconds between the real start time
> > > >   and the start time reported by ps. Now, 24 hours since reboot
> > > >   the difference is 10 seconds."
> > > >
> > > > The calculation used is:
> > > >
> > > >   now - uptime + time_from_boot_to_process_start
> > >
> > > Start-time and uptime is using different source. Looks like the
> > > jiffies was added bogus lost counts.
> > >
> > > quick hack. Does this change the behavior?
> >
> > Where did this all end up?  Complaints about wandering start times are
> > persistent, and it'd be nice to get some fix in place...
> >
> > Thanks.
>
>
> Seems my analysis of the problem wasn't perceived as such.
>
> The problem is that in the above calculation 
>
>   now - uptime + time_from_boot_to_process_start
>
> "uptime" currently is an ntp-corrected precise time, while 
> "time_from_boot_to_process_start" just is the free-running "jiffies"
> value.

I see you think you have the solution, but I guess I am just dense here.  May be 
you could help me to see the error of my ways.  Here is my thinking:

"now" is from gettimeofday() and as such is ntp corrected.
"uptime" is also corrected.  In fact it is "now" + "wall_to_monotonic".  And 
"wall_to_monotonic" is _only_ changed by do_settime() when the clock is set.
"time_from_boot_to_process_start" is the same as "start_time" restated in 
seconds, i.e. it is a constant.  So, either one or more of the above assumtions 
is wrong, or  somebody is twiddling the clock.  Otherwise I don't see how the 
start time can move at all.
> The problem is easily reproducible for me. It goes away if the change
> that rebased /proc/uptime on posix monotonic time and my followup patch to 
> fix the resulting rounding issues in jiffies64_to_clock_t() are backed out 
> with the following patch.

--
George Anzinger   george@xxxxxxxxxx
High-res-timers:  http://sourceforge.net/projects/high-res-timers/
Preemption patch: http://www.kernel.org/pub/linux/kernel/people/rml

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